The prior art reveals various methods for decreasing the distortion which results during the precipitation hardening of formed parts produced from copper beryllium alloys. Unfortunately, these prior art methods are minimally effective and often fail to control the resultant distortion to commercially acceptable degree. Additionally, the prior art methods yield inconsistent non-reproducible results. These alloys are used in electrical connectors where consistent dimensional and mechanical properties in the finished product are important.
Basically, all prior art methods for producing formed parts from copper beryllium alloys include the combination of the following sequence of procesing events: preparing a copper beryllium melt; casting the melt; hot working the cast copper beryllium; solution annealing the copper beryllium; cold working the solution annealed copper beryllium. As mentioned above, various methods have been developed in an attempt to control the distortion experienced in this processing sequence.
In this connection, reference is made to the methods disclosed in Goldstein U.S. Pat. No. 4,425,168, McClelland U.S. Pat. No. 4,394,185, Wickle U.S. Pat. No. 4,179,314, Shapiro U.S. Pat. No. 3,882,712, Britton U.S. Pat. No. 3,658,601, the article entitled "Residual Stresses in Copper-2% Beryllium Alloy Strips", authored by K. E. Amin and S. Ganesh, Experimental Mechanics, Dec. 1981, page 474, and the articles entitled "A Technique for Predicting Distortion and Evaluating Stress Relief in Metal Forming Operations", authored by K. E. Amin and R. M. Rusnak, Journal of Metals, Feb. 1981. "Stress Relaxation in Bending of Copper Beryllium Alloy Strip", authored by A. Fox in Journal of Testing and Evaluation, Vol. 8, No. 3 May 1980, and "Schrumpfung and Verzug bein Aus Harten von Kupfer Beryllium Legierungen", authored by H. Kreye, H. Noeka and F. Terlinde in Metall, 29 Jahrgang, Nov. 1975.
The methods disclosed in these prior art sources are only partially successful in eliminating distortions in finished products. Amin and Ganesh have correctly identified residual stresses as one of the sources of distortion. Amin and Ganesh have also shown that a high rolling reduction of copper beryllium strip results in tensile residual stresses near the surface of the strip and compressive residual stresses at the center of the strip while low rolling reductions result in the opposite location of these stresses within the strip. The ability to create a reversal in the patterns of tensile and compressive residual stresses will be used to define the differences between heavy and light reductions. Kreye et al have shown that too severe a reduction, such as would result from hammer gorging, will remove any such residual stress patterns in beryllium copper.
The McClelland and the Britton patents comprehend the importance of relieving residual stresses prior to the forming operation by the incorporation of a pre-aging technique. However, they fail to realize that in a thermal treatment such as their pre-aging technique two reactions occur simultaneously. On the one hand thermal treatments such as pre-aging promote the nucleation and growth of the precipitates formed during precipitation hardening. On the other hand these treatments also reduce the magnitude of the existing cold working and residual stress patterns that affect the precipitation hardening. Fox has shown that in beryllium copper, stress relaxation and precipitation hardening can occur simultaneously at the same temperature. The recognition of these competing mechanisms is critical in the develoment of reproducible softening and hardening techniques and the effects thereof on the reproducibility of the formed parts. All thermal treatments must utilize those combinations of times and temperature which relieve or decrease the magnitude of residual stresses before the formation of precipitates become dominant.
None of the prior art teachings recognize that the rates at which the nucleation and growth of precipitates occur are different when the metal matrix is aged under tensile residual stresses as opposed to compressive residual stresses. It has been shown that the formation of the coherent .gamma. (gamma) precipitates create shrinkage in the matrix as well as in the precipitates themselves. Based on energy considerations for the diffusion of the Be atoms towards the .gamma. (gamma) precipitate nuclei and on data obtained by the applicants, it is apparent that more precipitates would be created where there are compressive residual stresses than where there are tensile residual stresses. Then there will be more shrinkage in the original compressive regions than in the original tensile regions. Lack of reproducibility in the original residual stress patterns means lack of reproducibility in the magnitudes of the shrinkages caused by thermal aging. This can explain why reducing residual stresses can decrease the non-reproducible distortion created by the aging of coherent precipitates. However, what has not been recognized by the prior art is the existence of precipitate patterns, created by these differences in the rate of their formation and the role that such precipitate patterns, formed by aging, play in the post hot forming, cold forming and solution annealing operations. If there is a precipitate pattern, then when the precipitates are given the normal short time anneal, there will be left a residual pattern of beryllium atoms and undissolved precipitates. This residual pattern could form the basis for the memory, which becomes evident on aging, that the formed part has of its thermal and mechanical history. This memory could be minimized by the formation of an opposite pattern created by the second reduction and annealing taught in our claims. It is recognized that by the time that non-coherent precipiates are formed, essentially all effective residual stresses have vanished.
Furthermore, to date there has been no application of the effects of light and heavy reductions to the controlling and leveling out of residual stresses within the alloy or to the modification of precipitate patterns left after an incomplete solution anneal.